Most data collected by sensor nodes which for example form a wireless sensor network, changes very slowly. Consequently, in many applications most data needs to be transmitted only once every few minutes or even every few hours.
Sensor nodes typically include a transmitter/receiver structure (i.e. a transceiver), a sensor, a sensor interface, an energy source like e.g. a battery and a controller. The sensor and the energy source may be integrated in the sensor node or be external to it.
There is a clear tendency towards the use of batteries with an ever increasing lifetime. To meet the tough goal of a 10-year battery life time with a one-time installation or energy harvest operation, a radio-frequency (RF) transceiver (which is by far the most power-hungry block in a sensor network) should have an extremely low duty cycle (i.e., the ratio of on-time to sleep time), to minimize the average power consumption. For example, the average power consumption of the RF transceiver should be lower than 1 μW in order to sustain for 10 years with a 100 mAh small battery.
There are two key limiting factors for reaching a minimum average power consumption and duty cycle: leakage current and the accuracy of the real-time counter. To address the leakage issue, a sophisticated power management and non-volatile memory have been widely explored. However, the real-time-counter accuracy has so far received little attention. If the real-time-counter has a frequency error of 1%, the lowest duty cycle is also limited to approximately 1%, because the transceiver needs to be enabled longer in order not to lose synchronization. For a transceiver with 10 mW peak power, the duty cycle need to be reduced to 10−5 to achieve an average power below 1 μW, this requires the real-time-counter accuracy to be approximately 10 ppm, i.e., an accuracy similar to that of a crystal oscillator (XO). Note the low-power RC-based real-time-counter typically has an accuracy in the order of 10000 ppm.
Further reducing the duty cycle does not lead to a reduction of the average power either when the settling time of the main crystal oscillator or the system processing overhead becomes a dominant factor.
In the art the use of a high precision external temperature compensated crystal oscillator has been proposed. Such a temperature-compensated XO may consume as much as 10 μA, i.e. 100 times more than an on-chip relaxation oscillator. An important drawback is that such a technique may be very expensive.
If a crystal oscillator is used to calibrate the real-time-counter, only the frequency offset is calibrated, but not the noise, temperature, or supply voltage induced variation.
Also in case the frequency is synchronized in the network level, only frequency drift is calibrated, but not the noise induced variation.
Hence, there is a need for a synchronization method which is ultra-low power in order to enable a long operation life time or energy harvesting based operation.